The present invention relates generally to communications, and more specifically to modulation and demodulation systems.
As the use of personal wireless radio communications systems, such as digital cellular telephones, operating in frequency bands above 900 MHz, continues to expand there are continuing demands to reduce the overall size and weight of hand-held battery operated terminals. Since much of the bulk and mass of such devices is contributed by a power source, typically a battery pack, reduction in size of the battery pack has a considerable impact on their overall size and weight. The size and mass of the battery pack for a given charge life is dictated largely by the operating voltage and power requirements of the wireless terminal. In order to reduce the size and weight, it is desirable to reduce both the required supply voltage and the power consumption of circuitry used in such devices.
In many communications systems, specifically wireless communications systems, an information signal is modulated onto a higher frequency carrier before being transmitted. In these systems, there is often a need at the receiver end for some form of frequency translation back to lower frequencies. A decision device is then used to recover the information contained in the lower frequency signal. The frequency translation is often performed using frequency mixers. Frequency mixers are important building blocks in transceiver design since the dynamic range and noise/distortion performance of a transmitter/receiver is determined, and often limited, by the first up-down-conversion mixer. There are several types of mixer design used in the industry, probably one of the more popular being the Gilbert mixer.
In a Gilbert mixer an emitter-coupled transistor pair is used to switch the current path between the inner and outer transistors of a collector-cross-coupled coupled quad; this switching creates a double-balanced mixing action. Gilbert mixers differ significantly from diode mixers. The Gilbert cell is an amplifier, so the mixer now has conversion gain rather than the approximately 7 dB loss of a passive mixer. The switching action of the mixer takes relatively little energy to initiate, so although the Integrated Circuit (IC) mixer must be DC biased, its total power budget is often less than that required for a passive mixer driven by a much higher-powered LO. The Gilbert cell is often less sensitive to loading, and doesn""t require any bulky transformers, making it an extremely convenient device to use.
The traditional Gilbert mixer has a structure which is based on the application of the well-known cascode structure (also known as a totem pole or tree structure), which, although providing a stable and approximately linear performance, has a disadvantage of requiring a relatively high voltage to operate.
In some situations a dual mixer configuration is used which comprises two pairs of quadrature mixers and a Local Oscillator (LO) power divider thereby yielding two sequential steps of frequency conversion using two separate LO signals. The dual mixer is typically used in image rejection mixers. The quadrature LO required may be generated by any of the methods known to those skilled in the art.
When the components are integrated onto a single integrated circuit (IC) there are several advantages over using discrete components; including: 1) the size and weight of the dual mixer is a fraction of the discrete components combined; 2) interconnections between components are eliminated, which leads to higher performance by eliminating the signal reflections associated with them; 3) by having both mixers in the same housing they will track better over temperature leading to improved performance through better matching; and 4) the price and parts-count of the system are reduced.
Typically, in designing analogue circuits, particularly those implemented in highly integrated technologies, a number of design compromises must be made in the selection of various parameter values, among them conversion gain, local oscillator performance, linearity, noise figure, port-to-port isolation, voltage supply and power consumption. An example of this compromise is found in the design of a xe2x80x98traditionalxe2x80x99 Gilbert dual mixer, where the current through the active mixer stages is used to drive the Radio Frequency (RF) amplifier stages. To achieve sufficient linearity in the active mixer stages, the current must be higher than one might wish to achieve a good noise performance in the RF amplifier stages. The design must end up with some compromise in the selection of one or more of these parameters. This type of decision becomes more critical in systems with limited power availability, either because the supply voltage must be lower than in earlier designs, or because the designer is also tasked with providing a circuit with the lowest possible power consumption. These conditions are frequently met in the domain of interestxe2x80x94Personal Communications Systemsxe2x80x94which has become dominated by requirements of increased battery life and lower component count and cost, all of which combine to drive the quest for a higher level of integration, despite lower battery voltages.
Other challenges in the design of a xe2x80x98traditionalxe2x80x99 Gilbert dual mixer include the reliance in the operation of the device on the close matching of the parameters of the transistors and other active and passive devices used in the generating of the I (In-phase) and Q (Quadrature-phase) signals and the mixing of the RF signal with the LO signal.
With the wide range of competitive modulation mechanisms and pressure on the limited bandwidth in any one frequency band allocated for cellular wireless, especially in North America, there is an increasing demand for multi-mode and multi-frequency designs for the cellular wireless market. The differing requirements on the receiver performance placed on it by the need to deal with frequencies over an octave apart means that it is desirable to vary some performance characteristics as the frequency changes, or even to optimise the power use for a single frequency. One such parameter is the conversion gain. Many existing mixer designs have fixed gain.
As modern Very Large Scale Integration (VLSI) technologies enable higher levels of integration in portable communication applications, high performance radio frequency circuitry is required to meet stringent performance specifications while operating from a low voltage supply. The folded Gilbert cell mixer is an application of current folding that allows Gilbert cell mixing operation at low voltages (below 1.8 volts).
The invention alleviates the aforementioned disadvantages and other disadvantages, and provides the following advantages:
Ability to work at lower voltages
Good matching between I and Q signals.
High linearity.
Decreases the switching noise in the active mixer by reducing the switching current.
Variability of conversion gain.
These advantages are accomplished through the following features taken singly or in suitable combinations:
The combining of xe2x80x98activexe2x80x99 and xe2x80x98passivexe2x80x99 mixers in a single circuit.
The use of single input RF transistors for the I and Q signals.
The injection of a bias current between the RF transistors and their associated mixers to reduce noise in the mixer switches, yet maintain sufficient current in the associated input amplifier circuits for correct operation.
The use of simple resistive elements to fix the active mixer biasing voltages for optimum linearity.
The invention can be applied to both single stage and dual mixers to give low noise performance and Automatic Gain Control (AGC). As devices become more sophisticated, and are able to measure their own performance in real-time, it is becoming practical to optimise their performance, if not during actual use, then at least during some part of the final factory testing. One such parameter is the linearity of the mixer.
For testing purposes, a prototype chip using the invention was designed, simulated, and then fabricated in a standard Silicon high-performance Complementary Metal Oxide Silicon (CMOS) technology process which allows integrated circuits to operate in applications at speeds above approximately 900 MHz.